Enhancing search capacity of global navigation satellite system (GNSS) receivers

ABSTRACT

Enhancing search capacity of Global Navigation Satellite System (GNSS) receivers. A method for searching satellite signals in a receiver includes performing a plurality of searches sequentially. The method also includes storing a result from each search of the plurality of searches in a consecutive section of a memory. Further, the method includes detecting free sections in the memory. The method also includes concatenating the free sections in the memory to yield a concatenated free section. Moreover, the method includes allocating the concatenated free section for performing an additional search.

TECHNICAL FIELD

Embodiments of the disclosure relate to a Global Navigation SatelliteSystem (GNSS) receiver.

BACKGROUND

A GNSS receiver determines its position using satellites. The GNSSreceiver processes a received signal to determine presence of signalsfrom various satellites. If presence of a signal from a satellite isdetected, then the satellite is determined as a “visible” satellite.After the visible satellites are determined, the GNSS receiver furtherprocesses the signals from the visible satellites to determine itsposition. Time required by the GNSS receiver to determine its positionis dependent on time consumed by the GNSS receiver in performing thesearch for the visible satellites. Hence, it is desired to minimize timeconsumed by the GNSS receiver in performing the search for the visiblesatellites and in giving a position fix.

An exemplary GNSS receiver that performs the search is illustrated inFIG. 1 (Prior Art). The received signal is received by an antenna 105and processed using a circuit 110. The circuit 110 can include a radiofrequency processing circuit that down-converts the received signal tointermediate frequency for further processing. An analog-to-digitalconverter (ADC) 115 converts output of the circuit 110 into digitalsamples. A filter and decimation block 120 filters the digital samplesto reduce sampling rate and to bring the received signal to baseband.The digital samples are then stored in a buffer 125. Correlators 130A to130N access the digital samples from the buffer 125 and storecorrelation results in a corresponding section in a memory 145. Forexample, a correlator 130A stores correlation result in a section 135Aof the memory 145. Each correlator and the corresponding section in thememory 145 correspond to a channel. Each channel is capable of searchingfor a signal from a particular satellite. The correlation results areindicative of visible satellites. Once a visible satellite is detected,a tracking channel 140 tracks the visible satellite to receive data andto track the position. However, searches corresponding to variouschannels are performed in parallel using several correlators leading tohigh cost. Further, each channel is allocated a section, in the memory145, required for worst case search leading to inefficiency. Moreover,searches corresponding to various channels may require different memorysizes based on mode of operation of the GNSS receiver and henceallocation of the section based on worst case search requirement leadsto wastage of the memory 145.

SUMMARY

An example of a method for searching satellite signals in a receiverincludes performing a plurality of searches sequentially. The methodalso includes storing a result from each search of the plurality ofsearches in a consecutive section of a memory. Further, the methodincludes detecting free sections in the memory. The method also includesconcatenating the free sections in the memory to yield a concatenatedfree section. Moreover, the method includes allocating the concatenatedfree section for performing an additional search.

An example of a method for searching satellite signals from a pluralityof satellites in a global navigation satellite system receiver includesallocating a plurality of sections of a memory for a plurality ofsearches. Each search of the plurality of searches is a 3-dimensionalsearch. The 3-dimensional search includes searching for a code for allcode phases for one Doppler hypothesis. The code corresponds to asatellite. The method also includes performing a plurality of searchessequentially. The method includes storing a result from each search ofthe plurality of searches in a corresponding allocated section of theplurality of sections of the memory. The method also includes detectingat least one of termination of at least one group of code phase searchesfor a particular search in the plurality of searches, and termination ofall code phases for the particular search from the plurality ofsearches. The method further includes detecting free sections in thememory based on detecting termination. Moreover, the method includesconcatenating the free sections to yield a concatenated free section ifthe free sections, in conjunction, are sufficient for performing anadditional search. Furthermore, the method includes allocating theconcatenated free section for performing the additional search.

An example of a receiver includes a correlator, a memory and a memorysqueezing unit in electronic communication with the correlator and thememory. The memory squeezing unit includes a plurality of statusregisters, a plurality of shadow registers, a plurality of sets ofconfiguration registers and a state machine. The state machine is inelectronic communication with the plurality of status registers, theplurality of shadow registers, and the plurality of configurationregisters to enable searching for presence of data in a received signalby processing the received signal, moving bits from the plurality ofshadow registers to the plurality of status registers based on theprocessing, and updating the plurality of configuration registers basedon the processing.

BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS

FIG. 1 illustrates a portion of a receiver, in accordance with priorart;

FIG. 2 illustrates a portion of a receiver, in accordance with oneembodiment;

FIG. 3 illustrates operation of a memory squeezing unit, in accordancewith one embodiment;

FIG. 4 illustrates memory allocation during noise early termination, inaccordance with one embodiment;

FIG. 5 illustrates state of a memory, in accordance with one embodiment;

FIG. 6 illustrates allocation of a memory, in accordance with oneembodiment;

FIG. 7 illustrates steps performed by a state machine, in accordancewith one embodiment;

FIG. 8 illustrates state of registers of a memory squeezing unit duringa search, in accordance with one embodiment;

FIG. 9 illustrates a linked list implementation scheme, in accordancewith one embodiment;

FIG. 10 illustrates a method for searching signals in a receiver, inaccordance with one embodiment; and

FIG. 11 illustrates a method for searching presence of signals from aplurality of satellites in a global navigation satellite systemreceiver.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 illustrates a portion 200 of a receiver. The receiver can be aGlobal Navigation Satellite System (GNSS) receiver or a GlobalPositioning System (GPS) receiver. Applications of the receiver include,but are not limited to, emergency and location based applications,geo-tagging, and geo-fencing. The receiver can be present in variouselectronic devices, for example a Personal Navigation Assistant (PNA) ora Portable Navigation Device (PND), a car navigation system, and amobile phone.

The portion 200 searches for visible satellites. The “visiblesatellites” are defined as satellites having line-of-sight path to thereceiver for communication. A satellite can be determined as a visiblesatellite by searching for and detecting presence of data from thesatellite in a received signal. The received signal is defined as asignal received by the receiver. Once a visible satellite is determined,the visible satellite is tracked at a resolution higher than that forsearching. Higher resolution indicates higher sampling rate. Trackingthe visible satellite at the higher resolution enables the receiver tofind position of the receiver with higher accuracy in three-dimensional(3D) position co-ordinates. The position of the receiver can then becommunicated to a user of an electronic device that includes thereceiver.

The portion 200 includes an antenna 205. The antenna 205 is coupled to aprocessing unit 210. The processing unit 210 is coupled to ananalog-to-digital converter (ADC) 215. The ADC 215 is further coupled toa filter-and-decimation component 220. The filter-and-decimationcomponent 220 is coupled to a buffer, for example a double buffer. Thedouble buffer includes a buffer 225A and a buffer 225B. An example ofthe double buffer is a ping-pong buffer. The double buffer is coupled toa multiplexer 230. The multiplexer 230 is coupled to a correlator 235.The correlator 235 is coupled to a memory processing unit 240 thatincludes a memory squeezing unit 250. The memory processing unit 240 isfurther coupled to a memory 245.

The antenna 205 receives and provides the received signal to theprocessing unit 210. The received signal includes data, also referred toas satellite signals, from one or more satellites.

Each satellite signal includes message data modulated by a code. Themessage data modulated by the code is transmitted on a high frequencycarrier. The message data includes, but is not limited to, informationon co-ordinates of the satellite, orbit of the satellite, synchronoustime information and health of the satellite. Examples of the codeinclude, but are not limited to, a gold code (C/A), a Barker code, apseudorandom code, and a Welsh code. In one example, when the satellitesignal is from a GPS satellite, the code of the satellite signal canhave a length of 1023 bits (also referred to as chips) and a period of 1millisecond (ms). In another example, when the satellite signal is froma GLObal NAvigation Satellite System (GLONASS) satellite, the code ofthe satellite signal can have a length of 511 bits/chips and a period of1 ms.

The code is used to identify presence of the satellite signal in thereceived signal. There is a time lag between transmission of thesatellite signal from the satellite and receipt of the satellite signalby the receiver. The time lag can be defined as an indication ofdistance between the satellite and the user, which is used fortrilateration calculations to compute location of the receiver. The codecan be shifted in order to identify presence of the satellite signal forthe time lag. Shifting also helps in determining magnitude of the timelag. Each shifted version of the code can be referred to as a “codephase”. Searching for the exact time lag is referred to as a “code phasesearch”.

In one example, the portion 200 searches for presence of satellitesignals from 32 satellites in the GPS constellation and hence there are32 codes. One code corresponds to one satellite. Each code is unique andorthogonal to the rest of the codes. The portion 200 identifies thesatellites through these respective unique codes. For example, a GLONASSsatellite, having a frequency division multiple access (FDMA) signalstructure, is identified by its frequency band which is different withrespect to other satellites.

The processing unit 210 includes a Low Noise Amplifier (LNA), adown-converter, and filtering circuitry. The LNA amplifies the receivedsignal and the down-converter then down-converts the received signal toan Intermediate Frequency (IF) signal. The ADC 215 converts output (theIF signal) of the processing unit 210 into digital samples.

The filter-and-decimation component 220 converts the IF signal tobaseband. The filter-and-decimation component 220 performs decimationoperation on the digital samples to decrease sampling rate. The digitalsamples obtained as output of the filter-and-decimation component 220are referred to as filtered samples. The filtered samples correspondingto one frame are stored in the buffer 225A or the buffer 225B. In oneexample, a frame is equal to a time period of 1 ms. When the filteredsamples in the buffer 225A are being processed, the filtered samplescorresponding to next frame are being stored in the buffer 225B and whenthe filtered samples in the buffer 225B are being processed, thefiltered samples corresponding to next frame are being stored in thebuffer 225A. The multiplexer 230 selects filtered samples from thebuffer 225A or the buffer 225B.

Various other circuits than those shown in FIG. 2 may be present toprocess the received signal before the filtered samples are obtained.

The portion 200 is capable of performing a plurality of searches,hereinafter referred to as “the searches”, sequentially. In one example,the searches can be performed sequentially using time divisionmultiplexing within a code frame. A search can be defined as a3-dimensional search including search for a code, search for the codefor various code phases, and search for the code for various code phasesfor one Doppler hypothesis. Due to relative movement of the satellitesand the user, there can be a Doppler shift in frequency of the receivedsignal. To account for the Doppler shift, the search is performed fordifferent Doppler hypotheses. A “Doppler hypothesis” is defined as anestimate of the Doppler shift in the frequency. Several such estimatesare considered.

The searches can be enabled by the correlator 235, the memory squeezingunit 250, and the memory 245.

The portion 200 is capable of generating various codes locally. Eachcode corresponds to a satellite. The correlator 235 enables a firstsearch by correlating the received signal with a first code for variouscode phases for a first Doppler hypothesis. The first search correspondsto a search for the satellite signal from a first satellite for thefirst Doppler hypothesis. Any satellite can be referred to as the “firstsatellite”. The search for the satellite signal from the first satellitecan be referred to as the first search. Any Doppler hypothesis can bereferred to as the “first Doppler hypothesis”. The first code isgenerated locally by the portion 200.

The correlator 235 correlates the filtered samples selected by themultiplexer 230 with the first code. The first code is shifted by onechip with respect to the filtered samples and correlation is thenperformed with shifted first code. The first code is shifted by twochips and correlation is then performed with shifted first code. Processof shifting the first code and correlating the filtered samples withshifted first code is continued until the first code is shifted by Nchips with respect to the filtered samples. Correlation of the filteredsamples with the shifted first code is referred to as a code phasesearch.

Output of the correlation with the first code is then stored in a firstsection of the memory 245. Similarly outputs of various correlationswith various codes are stored in different sections of the memory 245.

In some scenarios, the satellite signal is weak and has lesser powerthan a thermal noise floor. In one example, in open sky conditions, thepower of the satellite signal is −130 dBm and that corresponding to thenoise floor is −111 dBm. The power of the satellite signal is thus 19dBm below the noise floor. To detect the presence of the satellitesignal in the received signal, signal-to-noise ratio (SNR) of thereceived signal needs to be improved. The SNR can be improved byperforming coherent integrations and non-coherent integrations. Acoherent integration includes summing the correlation resultcorresponding to a code phase across multiple frames after determiningthe Doppler shift through the Doppler hypotheses and removing theDoppler shift for particular Doppler hypothesis. The coherentintegration includes integrating I and Q components of the receivedsignal separately. A non-coherent integration includes accumulatingabsolute values of accumulated data from the coherent integration.Accumulated data from the coherent integration is referred to as “preddata” or “pred result” or “coherent data” or “coherent result”.Accumulated data from the non-coherent integration is referred to as“postd data” or “postd result” or “non-coherent data” or “non-coherentresult”.

For example, the first code has 1023 chips. All code phase searches ofthe first code are performed in every 1 ms period of 1 second (s).Correlation results corresponding to each code phase search in 1 msperiods are summed over 1 s. For perfect correlation of a code phase,the correlation result corresponding to the code phase and summed overis will be a maximum value that will exceed a predetermined threshold(first threshold). But in case there is no alignment between the firstcode generated by the portion 200 and that of the satellite signal, forthe code phase and the first Doppler hypothesis, then the correlationresult summed over 1 s will not be a maximum value as the correlationresult will not cross the predetermined threshold. The correlationresult hence resembles random noise.

If correlation values corresponding to all code phase searches summedover 1 s do not exceed the first threshold then the first search isterminated.

The correlator 235 is time-shared between correlations corresponding toeach search. The correlator 235 performs correlations corresponding tothe first search, and corresponding coherent data and non-coherent dataare then stored in the first section of the memory 245. The coherentdata and the non-coherent data together are referred to as a “result” ofthe first search. Similarly, the correlator 235 performs correlationscorresponding to a second search and corresponding coherent data andnon-coherent data are then stored in a second section of the memory 245.The memory 245 thus enables searching for presence of the satellitesignals in the received signal by storing the coherent data and thenon-coherent data of the correlations corresponding to the searches.

Initially, before searching starts, various sections in the memory 245are allocated for performing the searches for the satellite signals. Insome embodiments, if no prior searching is done then the sections ofidentical size can be allocated for each search.

In some embodiments, allocation is based on mode of operation of thereceiver. For example, when the receiver is in a hot start mode or inassisted GPS mode, the receiver has some prior information correspondingto the first satellite. Hence the first search may not includecorrelations corresponding to all the code phases. The mode of operationof the receiver can be the hot start mode when the receiver is poweredOFF and ON within a short span of time, for example one minute. Thefirst section of the memory 245 allocated for the first search can thenbe smaller as compared to sections allocated for other searches.

The memory squeezing unit 250 communicates electronically with thecorrelator 235 and the memory 245. The memory squeezing unit 250allocates the sections of the memory 245 for the searches. Thecorrelator 235 then in conjunction with the memory 245 and the memorysqueezing unit 250 performs the searches.

After some time from start of the searches, a few searches may beterminated due to detection of absence of a satellite signal, alsocalled a noise early termination, or due to presence of a satellitesignal, also called a signal early termination. Termination orcompletion of the few searches can leave intermediate sections of thememory 245 unused. The unused sections are referred to as free sections.It may also happen that instead of an entire search across all codephase searches for a particular satellite being terminated, a few codephase searches are terminated. In case of termination of a few codephase searches, an entire section of the memory 245 allocated for thecorresponding search may not be free but a few locations in that sectionmay become free. The few locations that are free are also referred to as“the free sections”. A “location” is defined as a portion of a memorywith an address. A group of one or more locations is termed a “section”.

The memory squeezing unit 250 then detects the free sections in thememory 245 and concatenates the free sections towards the end of thememory 245 to yield a concatenated free section. The concatenated freesection is then allocated for performing an additional search. Theadditional search is defined as a search other than the searches alreadyperformed. The search or searches to which the concatenated free sectionis allocated are referred to as the additional search or additionalsearches.

In one example, the additional search can include a search for asatellite signal from a satellite that was not considered in thesearches already performed. In another example, the additional searchcan include a search for the satellite signal from the first satellitefor a second Doppler hypothesis. Selection of the searches that need tobe performed can be done based on requirements. Various algorithms basedon which searches need to be selected can be included as firmware in thereceiver. In one example, N number of searches considered for processingsequentially can be selected based on N satellites. One search from Nsearches corresponds to one satellite. In another example, N number ofsearches considered for processing sequentially can be selected based onN Doppler hypotheses. One search from N searches corresponds to oneDoppler hypothesis. Two searches from N searches can include searchingfor the same code but for different Doppler hypotheses. It is noted thatvarious combinations of the Doppler hypotheses and the codes can be usedfor the searches.

The correlation results stored in the memory 245 are processed todetermine presence of the satellite signal. For example, asignal-to-noise ratio (SNR) is calculated based on the correlationresults stored in the memory 245 and checked against the firstthreshold. If the SNR exceeds the first threshold then presence of thesatellite signal is confirmed and the corresponding satellite isdetermined as a visible satellite. The portion 200 can then fix to andstart tracking the visible satellite to determine the position of thereceiver.

The memory squeezing unit 250 includes various elements for detectingthe free sections, concatenating the free sections and allocating theconcatenated free section to the additional search. The memory squeezingunit 250 including the elements is explained in detail in conjunctionwith FIG. 3.

Referring to FIG. 3 now, the memory squeezing unit 250 can be a hardwarecontroller that controls operation of the memory 245 and the correlator235. The memory squeezing unit 250 includes various registers and gates,and hosts various algorithms to perform the searches in conjunction withthe memory 245 and the correlator 235. The algorithms and hardwareconfiguration of the memory squeezing unit 250 are now explained inconjunction with FIG. 3 to FIG. 9.

In some embodiments, the memory squeezing unit 250 can be a part of thecontroller, for example the memory processing unit 240, for performingspecified functions.

The memory squeezing unit 250 detects free sections in the memory 245 bydetecting at least one of termination of at least one group of codephase searches for a particular search in the searches, and terminationof all code phases for the particular search from the searches. In otherwords, the memory squeezing unit 250 detects free sections in the memory245 by detecting termination of any number of searches or by detectingtermination of any number of code phase searches or a combination ofboth. Any search from the searches can be referred to as the firstsearch or the second search.

Noise Early Termination Algorithm

The memory squeezing unit 250 hosts a noise early termination algorithmwhich is now explained. In order to enable faster searches and utilizethe memory 245 to maximum extent, the memory squeezing unit 250calculates SNR of the satellite signal every few milliseconds (ms). TheSNR is calculated by summing the correlation results of an ongoing codephase search. The correlation results are summed over N ms and the sumis compared with a threshold (second threshold). Value of N can bedetermined based on the noise early termination algorithm. The secondthreshold varies with inclusion of every new correlation result in thesum. The magnitude of the second threshold varies and is represented asa predetermined threshold slope. Based on comparison of the SNR with thesecond threshold, a determination is made by the memory squeezing unit250 regarding dropping the code phase search. When the SNR of thesatellite signal is below the second threshold then the code phasesearch is dropped, and the coherent data and the non-coherent data ofthe code phase search are not stored in the memory 245. The dropping ofthe code phase search before termination of a dwell due to detection ofnoise is known as noise early termination. The dwell is indicative ofthe number of coherent integrations and the number of non-coherentintegrations. For example, (M, N) dwell indicates M coherentintegrations and N non-coherent integrations. The M coherentintegrations and N non-coherent integrations depend on sensitivity ofthe code phase search.

Signal Early Termination Algorithm

When the SNR is found to be much greater than the second threshold thenthe search for the satellite is dropped since there is an indication ofthe presence of the satellite signal. The SNR can be determined to bemuch greater than the second threshold by checking whether the SNRexceeds the second threshold by a predefined value. The dropping of thesearch before termination of the dwell due to detection of the satellitesignal is known as signal early termination. In case of the signal earlytermination, the entire search is terminated as compared to the noiseearly termination where individual code phases or groups of code phasescan be dropped from a particular search.

Hence, some free sections in the memory 245 can be present due to noiseearly terminations or signal early terminations or both.

Code Phase State Maintenance Algorithm

The memory squeezing unit also hosts an algorithm for maintaining statesof the code phases. This algorithm will now be explained. The memorysqueezing unit 250 divides the code phases into groups as shown in FIG.4. In one example, a resolution of 2 samples per second (sps) with 2046code phases corresponding to 1023 chips of the code is considered. The2046 code phases are divided into 64 groups with 32 code phases in eachgroup. The 64 groups include a group 405A1 to a group 405A64. 2046 codephases correspond to one search. The noise early termination algorithmrunning in the memory squeezing unit 250 determines that one or moregroups of the code phases have to be dropped when the respective SNRs donot cross the second threshold. The memory squeezing unit 250 maintainsinformation on whether each group of the code phases has been dropped ornot.

The memory squeezing unit 250 includes one or more status registers, forexample a status register 410A, and one or more shadow registers, forexample a shadow register 415A. One shadow register is presentcorresponding to one status register. A pair of a status register and ashadow register can correspond to one search.

One bit of the status register 410A corresponds to one group. Eachstatus register has 64 bits (as shown in FIG. 4) corresponding to the 64groups of code phases. The bits are called valid bits. Based on resultsof the noise early termination algorithm, the memory squeezing unit 250updates the valid bits. The status register 410A indicates the validbits in a current time period or current ms. The shadow register 415Aindicates the valid bits for processing the search in a next time periodor next ms. The shadow register 415A in conjunction with the statusregister 410A indicates the presence of free sections in the memory 245for processing. The shadow register 415A also has 64 bits (as shown inFIG. 4) corresponding to the 64 groups of code phases.

The memory squeezing unit 250 uses samples from adjacent groups of codephases along with samples of a group of code phases to determine whetherthe group of code phases has to be dropped. For example, for a group405A2, correlation results of the group 405A2 can be considered alongwith 10 correlation results of each adjacent group, for example a group405A3 and the group 405A1. The results from the group 405A1 and thegroup 405A3 help in identifying peak signal values at edges of the group405A2. When the valid bit corresponding to the group of code phasesindicates that the group of code phases has been dropped, the memorysqueezing unit 250 shifts the code by 32 bits. The group of code phasesthat is dropped is not used for further processing. Likewise, the 32code phases that are dropped are not used for further processing.

An exemplary dropping of a group 405A6 is illustrated in FIG. 5. The SNRcorresponding to the group 405A6 does not meet the second threshold andhence needs to be dropped. X axis in FIG. 5 represents the groups and Yaxis represents the SNR.

If groups adjacent to a group are dropped then 32 correlation results ofthe group can only be used. For example, for a group 405A63 only 32correlation results of the group 405A63 are considered.

Base Address Computation Logic for Searches

The memory squeezing unit 250 also uses the valid bits to determine thenumber of free sections in the memory 245 corresponding to each searchand the total number of free sections in the memory 245. The memorysqueezing unit 250 uses the total number of free sections to generateaddresses of locations in the memory 245 where the correlation resultshave to be stored. For example, for concatenating the free sectionstowards the end of the memory 245 and yielding a concatenated freesection, the memory squeezing unit 250 needs to shift the correlationresults such that the correlation results are stored contiguously andthere are no free sections between the correlation results. Storing thecorrelation results contiguously includes shifting the correlationresults from non-consecutive sections in the memory 245 to consecutivesections in the memory 245. The memory squeezing unit 250 thus has togenerate the addresses of the locations in the memory 245 to which thecorrelation results have to be shifted. One address corresponds to onecorrelation result of one search.

A section of the memory 245 corresponding to a search includes a firstset of locations for storing coherent data of the search and a secondset of locations for storing non-coherent data of the search. Theaddress of each location in the first set is generated according to abase address of the coherent data. The base address of the coherent datais defined as the address of a first location in the first set. Theaddress of each location in the first set is generated by adding anoffset to the base address of the coherent data. Similarly, the addressof each location in the second set is generated according to a baseaddress of the non-coherent data. The sections of the memory 245corresponding to different searches have different base addresses forthe coherent data and for the non-coherent data, and the address of eachlocation in each section is generated from the corresponding baseaddress by adding offsets.

The memory squeezing unit 250 hosts logic for computing and generatingthe base addresses for coherent data and for non-coherent data. Further,the memory squeezing unit 250 updates the base addresses for thecoherent data and for the non-coherent data for each search. For purposeof explanation, sections of the memory 245 can be allocated startingfrom top towards bottom. As the sections of the memory 245 aredetermined to be free and sufficient for starting the additional search,the free sections can be moved towards the bottom of the memory 245, andthe base addresses are updated for each search based on the freesections in the memory 245 by performing vector addition of the validbits. For example, for an ongoing search, vector addition of the validbits for all searches performed before the ongoing search is done toupdate the base addresses for the coherent data and the non-coherentdata in sections corresponding to the ongoing search. An ongoing searchis defined as a search that is being performed. The section of thememory 245 corresponding to the ongoing search is defined as an ongoingsection.

Memory Allocation and Processing of Searches

An example of allocation of the memory 245 is now illustrated inconjunction with FIG. 6. Allocation of the memory 245 is illustratedusing states “A”, “B”, “C” and “D”. Referring to state “A” in FIG. 6,the memory 245 is completely filled by results of the searches. Inillustrated example, N sections are allocated to N searches where eachsection can be used for storing a maximum possible number of correlationresults. Each section includes two sets of locations, the first set forstoring the coherent data and the second set for storing thenon-coherent data.

For each search, due to noise early termination, one or more groups ofcode phases can be determined to be dropped. The coherent data isconcatenated immediately, from next ms after determination of droppingof the groups. Referring to state “B” in FIG. 6, a few locations of thefirst set of locations for storing coherent data corresponding to eachsearch are free. In the illustrated example, the starting address of thefirst set of locations corresponding to each search does not change.Shrinking of the coherent data happens at the boundary of the searches.State “B” of the memory 245 illustrates the shrinking (highlighted ingray).

State “C” of FIG. 6 illustrates shrinking of the non-coherent data. Theshrinking of the non-coherent data is done in a next non-coherent(Postd) data frame. A non-coherent data frame is defined as a period oftime after which non-coherent integration is performed. Movement ofcoherent and non-coherent correlation results are performed along withthe coherent integrations and the non-coherent integrations. Further, noextra power is consumed while shifting the coherent data and thenon-coherent data within similar boundaries for the base addresses.Values of the bits of the shadow register 415A are not changed until thenext non-coherent data frame while values of the bits of the statusregister 410A are updated in the current non-coherent data frame. Theshadow register 415A thus helps in determining if groups of code phasesare dropped. The free locations in the first set of locations and thesecond set of locations can also be referred to as free sections. Whenfirmware in the memory squeezing unit 250 determines that the freesections when concatenated are sufficient to start the additional searchthen the firmware issues a command to the memory squeezing unit 250 toconcatenate the free sections. In some embodiments, the firmware canissue the command after every few milliseconds. The base addresses arenow updated to move the coherent data and the non-coherent data tocontiguous locations, and to concatenate the free sections. Aconcatenated free section 605 of the memory 245 is illustrated in state“D” of FIG. 6. The concatenation of the free sections to enable start ofan additional search is now explained in detail.

For concatenation of the free sections, the memory squeezing unit 250moves the coherent data and the non-coherent data to free locations inthe sections of the memory 245 above the ongoing section. Theconcatenation can be implemented by the memory squeezing unit 250 usinga hardware state machine (state machine) that is activated upon receiptof the command from the firmware. The state machine can be run aftercorrelations for samples of a few milliseconds or frames have been done.In some embodiments, the memory 245 is not accessed when the statemachine is run. The steps implemented using the hardware state machineare as follows. The steps are also illustrated in FIG. 7

Step 705 Take a search as an input. If the search is for the first timeand no prior search has been done then go to step 730 else go to step710.

Step 710 Perform vector addition for valid bit registers of all previoussearches. Vector addition for valid bit registers of the ongoing searchindicates total locations to be moved within the section for the ongoingsearch whereas vector addition for all the previous searches indicatesthe total address offset for data movement for the ongoing search.

Step 715 Update the base addresses for the coherent data and for thenon-coherent data.

Step 720 Move the coherent data to a new address offset.

Step 725 Move the non-coherent data to a new address offset.

Step 730 If any other search is left then go to step 710.

FIG. 8 illustrates the state of the registers of the memory squeezingunit 250 during a search. In illustrated example, a (5, N) dwell isconsidered. (5, N) dwell indicates 5 coherent integrations and Nnon-coherent integrations. The coherent integration is performed oncorrelation results of the search every ms and the non-coherentintegration is performed every 5 ms, N times.

The status register 410A and the shadow register 415A for the search arealso illustrated. The bits of the status register 410A and the shadowregister 415A at various stages are shown. In the illustrated example,each stage corresponds to 5 ms.

The status register 410A indicates valid bits for 64 groups of codephases corresponding to the search. In the illustrated example, thememory squeezing unit 250 detects a noise early termination. The memorysqueezing unit 250 identifies group 405A62, group 405A63 and group405A64 as groups to be dropped for the search. The shadow register 410Ais updated accordingly and the valid bits (highlighted in gray in “Stage2”) corresponding to the group 405A62, the group 405A63 and the group405A64 indicate that the group 405A62, the group 405A63 and the group405A64 are to be dropped. The valid bits from the shadow register 415Acan be used, from the ms next to the ms in which the valid bits areupdated, for squeezing coherent data. Also, the status register 410A isupdated based on the shadow register 415A after moving the non-coherentdata in the next non-coherent data frame as shown at boundary of “Stage2” and “Stage 3”. The status register 410A is updated by moving bits ofthe shadow register 415A to the status register 410A.

In the illustrated example, when the memory squeezing unit 250determines that the free sections of the memory 245 due to code phasesearch dropping are enough to start an additional search, the statemachine starts concatenation of the free sections upon receipt of acommand from the firmware (FW). In the illustrated example, theconcatenation starts in “Stage 4”.

Linked List for Searches

The memory squeezing unit 250 also hosts a linked list scheme tosequence searches and to allocate the concatenated free section to theadditional search. The memory squeezing unit 250 includes one or moresets of configuration registers as shown in FIG. 9. One set correspondsto one search. The linked list scheme maintains a pointer scheme for thesets of configuration registers. For example, when a “search 2” for a“satellite 2” is dropped then a section of the memory 245 correspondingto the “search 2” is freed. The linked list scheme links a configurationregister set 905A1 of “search 1” to a configuration register set 905A3of “search 3”. The linked list scheme also links a configurationregister set 905AN of a last search in the searches to a configurationregister set 905A2. The linked list scheme maintains an allocation ofsections of the memory 245 and of the configuration registers to thesearches in a sequence, thus enabling movement of data in the memory 245in one direction.

When the concatenated section is allocated to the additional search, thelinked list scheme links the configuration register set of the lastsearch, for example 905AN (the last search is the search correspondingto a last non-free section in the memory) to a configuration registerset of the additional search. The configuration register sets areupdates based on the allocation.

The sequencing of searches and the allocation of the concatenated freesection can be referred to as processing a sequence of searches. Thissequence is an output of the memory squeezing unit 250. Inputs to thememory squeezing unit 250 include non-coherent integration results(non-coherent data), status of a dwell in a search, a search beingprocessed (search number), and an enable signal. The memory squeezingunit 250 generates control bit outputs or updates the status ofregisters to be used by the correlator 235 or by the algorithms of thememory squeezing unit 250.

Outputs of the memory squeezing unit 250 (these outputs are control bitoutputs) include a local code phase jump bit, a memory address offsetgeneration bit, and a post processor control bit. The local code phasejump bit indicates that the code corresponding to a search beingprocessed has to be jumped by 16 phases (shifted by 16 chips). The“memory address offset generation” bit indicates an offset to be addedto the base address for the coherent data or to the base address for thenon-coherent data to generate address of the location where acorrelation result of the search being processed has to be stored.

Outputs of the memory squeezing unit 250, indicative of status stored inthe registers, include an update of a base address for the coherent datafor the search being processed, an update of a base address for thenon-coherent data for the search, a processing sequence of searches, andcode phases dropped due to noise early termination or signal earlytermination.

The memory squeezing unit 250 enables the portion 200 to allocatesections of the memory 245 of different sizes to different searches asper requirement. For example, the size of a section of the memory 245required for a search in hot start mode of the receiver or assisted modeof the receiver is small than that required for cold start mode. Hotstart mode can be defined as the mode when the receiver already hasprior information available from a cellular network server. In the hotstart mode, the receiver has stored almanac information for thesatellite. The information can include position coordinates of thesatellite. Hence, the receiver has information giving the rough positionof the receiver and does not need to perform as many correlations aswhen the receiver does not have any such information. Since there arenot many correlation results to be stored in the hot start mode, thesize of the section of the memory 245 that must be allocated to thesearch can be less than that otherwise. Hence, the sizes of the sectionsallocated for different searches can be different and thus the memory245 can be utilized more efficiently than in a case where the sizes ofthe sections allocated for different searches are fixed. Search capacityof the portion 200 depends on the size of the memory 245 and thecomputation capability of the correlator 235. The search capacity can bedefined as the number of searches that can be performed by the portion200 in a fixed time interval.

Referring to FIG. 10 now, a method for searching satellite signals in areceiver is illustrated.

A plurality of sections of a memory, for example the memory 245, isallocated for performing a plurality of searches. The allocation can bebased on the mode of operation of the receiver and the algorithmssupported by the receiver. One section corresponds to one search.Different sections can be of different sizes. Each section can furtherinclude two sub-sections. The two sub-sections include a sub-sectioncorresponding to coherent integration results and a sub-sectioncorresponding to non-coherent integration results. The allocation can beconsidered as a part of performing the searches.

At a step 1005, the searches are performed sequentially. Performing eachsearch includes searching for presence of a satellite signal from adifferent one of a plurality of satellites in a received signal.Examples of the satellites include GPS satellites, Galileo satellites,GLONASS satellites, and Wide Area Augmentation System (WAAS) satellites.

In one embodiment, there are 32 GPS satellites and the receiver searchesfor the presence of one satellite signal from each satellite. When thepresence of a satellite signal is identified by the receiver, asatellite corresponding to the satellite signal is said to be visible tothe receiver. The receiver then maintains a fix to the satellite forreceiving position coordinates of the satellite. These positioncoordinates enable calculation of the position of the receiver.

The 32 satellite signals are transmitted by the 32 satellites. Everysatellite signal includes a code of the corresponding satellite thatrepeats every millisecond. The received signal is converted into digitalsamples and then filtered. The filtered samples are correlated withvarious codes sequentially. The codes are generated at the receiver.Each code corresponds to one satellite.

Each search includes correlations corresponding to one code, one Dopplerhypothesis, and all code phases. The code phases are used to determinetime lag between transmission of a satellite signal from a satellite andreceipt of the satellite signal by the receiver. The Doppler hypothesisis used to account for an offset in frequency of the code correspondingto a Doppler shift in frequency of the received signal, due to relativemotion between the satellite and the receiver. For detection of visiblesatellites and for good accuracy, the correlations are performed everymillisecond and results of the correlation are integrated over severalmilliseconds. Integration includes coherent integration which includessumming the results over several milliseconds or frames. Integrationalso includes non-coherent integration which includes squaring multipleresults of the coherent integration and summing the squares of theresults.

At step 1010, a result of each search is stored in a consecutive sectionof the memory. The result includes coherent and non-coherent integrationdata which is stored in the memory.

At step 1015, free sections are detected in the memory allocated forperforming the searches. The free sections can be detected in variousways. Two examples of detecting the free sections are now explained:

EXAMPLE 1

Detecting the free sections includes detecting termination of any numberof searches, for example detection of termination of a first search fromthe searches. The termination of the first search is detected if a SNRof the received signal is less than the second threshold. The SNR of thereceived signal is determined to be less than the first threshold if thesum of the correlation results after the time period or finish of dwell,is less than the first threshold. The finish of the dwell indicates thatthe coherent integrations and the non-coherent integrations arecomplete. The termination can also be detected if the SNR is well abovethe first threshold and hence presence of a satellite signal from asatellite is detected. Alternatively, the termination of the firstsearch can be due to finish of dwell. The section allocated for thefirst search is then determined as a free section. The termination ofthe first search is also detected if a predetermined time of the firstsearch for a particular sensitivity is completed in absence of adeterministic decision. The detection of absence or presence of thesatellite signal is referred to as the deterministic decision.

EXAMPLE 2

Detecting the free sections includes detecting termination of any numberof code phases in the searches, for example termination of a code phasesearch in a second search from the searches. The code phase searchcorresponds to a group of code phases. In one example, the group of codephases includes 32 code phases. Detecting termination of the code phasesearch includes comparing the sum of the correlation result beforefinish of dwell with the second threshold and terminating the code phasesearch if the sum of the correlation result before finish of the dwellis less than the second threshold. The code phase search is explained indetail in conjunction with FIG. 3. The locations of the memory thatbecome free due to the termination of the code phase search are referredto as the free sections.

At step 1020, the free sections are concatenated to yield a concatenatedfree section. The free sections are concatenated by shifting coherentand non-coherent integration results of one or more correlationscorresponding to the searches from one or more non-consecutive sectionsin the memory to one or more consecutive sections in the memory. Theshifting and concatenation has been explained above in detail inconjunction with FIG. 3 and FIG. 6. The free sections are concatenatedif the free sections, in conjunction, are sufficient for enabling anadditional search.

At step 1025, the concatenated free section is allocated for performingthe additional search. The concatenated free section is allocated to theadditional search if the concatenated free section is determined to besufficient to store integration results corresponding to the additionalsearch. Further, one or more sets of configuration registers (forexample as shown in FIG. 9) are assigned to the searches. The sets ofconfiguration registers initiate setup of the concatenated free sectionfor the additional search and enable coordination of data transfer toand from the concatenated free section while the additional search isbeing performed. The allocation of the concatenated free sectionincludes linking a set of configuration registers corresponding to theaddress of a last non-free section of the memory to another set ofconfiguration registers corresponding to the starting address of theconcatenated free section of the memory. Linking of the sets ofconfiguration registers can be implemented through a linked list. Thelast non-free section corresponds to a last search that is active fromthe searches being processed. The linking has been explained above indetail in conjunction with FIG. 3 and FIG. 6.

In one embodiment, the concatenated free section can also be allocatedfor tracking the visible satellite when presence of the satellite signalcorresponding to the visible satellite is identified in the inputsignal.

Referring to FIG. 11 now, a method of searching satellite signals from aplurality of satellites in a global navigation satellite systemreceiver, for example the receiver that includes the portion 200, isillustrated.

At step 1105, a plurality of sections of a memory are allocated for aplurality of searches. The sections can be allocated based on the modeof operation of the receiver and the algorithms supported by thereceiver. In some embodiments, if no prior searching is done then thesections allocated for each search can be of identical size. One sectioncorresponds to one search.

A signal received by the receiver includes the signals, also calledsatellite signals, from the plurality of satellites. In one embodiment,the received signal includes signals from 32 satellites. The position ofthe receiver can be calculated based on information in the signals. Theplurality of searches is performed sequentially until detection in thereceived signal of the presence of the desired number of satellitesignals for determining the position of the receiver.

At step 1110, the searches are performed sequentially. The searchesdetermine the presence of one or more of the satellite signals in thereceived signal. Each search is performed by correlating the receivedsignal with a code from a plurality of codes generated at the receiver.One code corresponds to one satellite.

Correlation is performed between the received signal and a first one ofthe codes. The first code is shifted by 1 chip and correlation isperformed again. The correlation is repeated until the first code isshifted by N chips, where N is a maximum number of times shifting can beperformed. Each shifted version of the first code is called a code phaseand accounts for a time lag between transmission of the signals from thesatellites and reception of the signals. The correlation correspondingto each code phase can also be called a code phase search.

Also, the correlation is performed for a Doppler hypothesis. The Dopplerhypothesis accounts for a shift in frequency in the received signal dueto relative motion between the satellites and the receiver. The searchis thus a 3-dimensional search that includes searching for the firstcode for all the code phases for one Doppler hypothesis. In oneembodiment, a next search in sequence can include searching for the codefor all the code phases for another Doppler hypothesis. In anotherembodiment, the next search can include searching for a second code forall the code phases for one Doppler hypothesis.

For good accuracy, correlation corresponding to a Doppler hypothesis, acode phase, and a code is performed every millisecond and results of thecorrelation are integrated over several milliseconds. Integrationincludes coherent integration which includes summing the results overseveral milliseconds or frames. The integration also includesnon-coherent integration which includes squaring multiple results of thecoherent integration and summing the squares of the results.

At step 1115, a result from each search is stored in a correspondingallocated section of the memory. The result includes coherent andnon-coherent integration data which are stored in the memory.

At step 1120, at least one of (a) termination of at least one group ofcode phase searches, for example two code phase searches, for aparticular search in the searches, and (b) termination of all codephases, for example a first search and a code phase search in a secondsearch, for the particular search from the searches is detected. Inother words, termination of any number of searches, or termination ofany number of code phase searches, or a combination of both is detected.The termination of the first search can be detected based on detectionof absence or presence of a satellite signal corresponding to the firstsearch. The detection of absence or presence of the satellite signal isreferred to as a deterministic decision. The termination of the firstsearch can also be detected if a predetermined time of the first searchfor a particular sensitivity is completed in absence of thedeterministic decision. The termination of the code phase search in thesecond search can be detected based on comparison of SNR of the receivedsignal with the second threshold. If the SNR is below the secondthreshold, a satellite signal corresponding to the second search isdetermined to be absent. If the SNR is above the second threshold, thesatellite signal corresponding to the second search is determined to bepresent.

At step 1125, free sections are detected in the memory based ondetection of termination or finish of dwell. The finish of the dwellindicates that coherent integrations and non-coherent integrations arecomplete. For example, due to termination of the first search or thecode phase search, integration results need not be stored in the memory,thus freeing up the sections.

At step 1130, the free sections in the memory are concatenated to yielda concatenated free section. The free sections are concatenated byshifting coherent and non-coherent integration results of one or morecorrelations corresponding to the searches from one or morenon-consecutive sections in the memory to one or more consecutivesections in the memory. The shifting and concatenation has beenexplained above in detail in conjunction with FIG. 3 and FIG. 8. Thefree sections are concatenated if the free sections, in conjunction, aresufficient for performing an additional search.

At step 1135, the concatenated free section is allocated for performinga search, for example the additional search, which is different than theplurality of searches.

Sections of different sizes can be selected based on the mode ofoperation of the receiver. This selection helps increase efficiency andoptimal use of the memory 245. The additional search can be performedwhen free sections in the memory 245, in conjunction, are sufficient forperforming the additional search. Hence, allocation of the free sectionsfor performing the additional search is independent of completion of allongoing searches. Further, since sections of different sizes can beallocated for performing the searches, the number of searches performedby the portion 200 can be high and hence, the search capacity of theportion 200 might also increase. Also, time taken by the portion 200 forthe searches and for the time-to-first-fix is minimized.

The portion 200 provides good performance in terms of power consumed.The portion 200 also occupies minimal area on a chip since the requiredsize of the memory 245 is reduced. Minimization of the size of thememory 245 also reduces the cost of the portion 200.

In the foregoing discussion, the term “coupled” refers to either adirect electrical connection between the devices connected or anindirect connection through one or more passive or active intermediarydevices. The term “signal” means at least a data signal, or othersignal.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the scope of thepresent disclosure, and that such modifications, alterations, andcombinations are to be viewed as being within the ambit of the inventiveconcept.

The foregoing description sets forth numerous specific details to conveya thorough understanding of embodiments of the present disclosure.However, it will be apparent to one skilled in the art that embodimentsof the present disclosure may be practiced without these specificdetails. Some well-known features are not described in detail in orderto avoid obscuring the present disclosure. Other variations andembodiments are possible in light of above teachings, and it is thusintended that the scope of present disclosure not be limited by thisDetailed Description.

What is claimed is:
 1. A method for searching satellite signals in areceiver, the method comprising: performing a plurality of searchessequentially; storing a result from each search of the plurality ofsearches in a consecutive section of a memory; detecting free sectionsin the memory including detecting termination of a first search from theplurality of searches, and in which detecting termination of the firstsearch further includes: detecting termination of one or more individualcode phase searches in a first search of the plurality of searches, eachcode phase search corresponding to a group of code phases, the group ofcode phases being obtained by shifting of a first code; determining asignal-to-noise ratio for the received signal at regular intervalsduring the first search; and comparing the signal-to-noise ratio with apredetermined threshold, considering a time of comparison; concatenatingthe free sections in the memory to yield a concatenated free section;and allocating the concatenated free section for performing anadditional search.
 2. The method as claimed in claim 1, whereinperforming a first search from the plurality of searches includessearching for data, from a first satellite of a plurality of satellites,in a received signal.
 3. The method as claimed in claim 2, whereinsearching for the data from the first satellite includes: correlatingthe received signal with a first code according to a first Dopplerhypothesis to obtain a correlation result; shifting the first code byone chip with respect to the received signal based on the correlationresult to yield a shifted first code; and correlating the receivedsignal with the shifted first code.
 4. The method as claimed in claim 2and further including: converting the received signal into digitalsamples; and performing the plurality of searches and the additionalsearch.
 5. The method as claimed in claim 1, wherein detectingtermination of the first search further includes detecting if apredetermined time of the first search for a particular sensitivity iscompleted in absence of a deterministic decision.